The Middle Game starts when players have no Free Moves but can only make Forced Errors. Since the goal is to kill all your opponents groups, you try to make the least worse move you can in this stage of the game. If you have any Enclosed Spaces then you should use these to maximum advantage. When you opponent plays a stone in this area do not immediately capture it. Rather it is better for you to play a stone that is adjacent to it. The reason for this is simple. If you capture it then your group only grows by 1 stone. If however you place a stone adjacent to your opponent in an Enclosed Space, then the next move you can capture his stone by joining the single stone and the Enclosed Space group of stones. If on the other hand he captures your stone while they are an Enclosed Spacesthen they have wasted a move (each capture they make they have to play another move, but any stones that are in an Enclosed Space are already a Dead Group.

Capturing

When one stone can make a Double Kill and the other stone can only make a single kill. Then to maximize your group size you should first capture with the single kill stone and then the other stone (which will also be a single kill after the first capture). So you always attack from the outside inwards.
First capture is made by the outside stone that can only make a single kill.
Then the second stone which initially could make a Double Kill but now will only make a single kill. By making single kills each time you maximize your group size.

In this situation you want to first make an Enclosed Space. This means you will get a group of Actual Size of 4 rather than 3.
So capturing on the outside starts to trap in the other enemy stone.
You create an Enclosed Space by not capturing but blocking his other stone in. Your enemy cannot capture this stone because it would be touching the group that has an Actual Size of 2. Notice that now you have a group with a Potential Size of 4.
At your lesuire you can create this group that has an Actual Size of 4. Alternatively you could capture on the yellow hex to create your group that has an Actual Size of 4. By capturing on the yellow hex your group is not as extended outwards. Without seeing the rest of the board it is impossible to say which hex is better to capture on.